This multidisciplinary program project aims at studying the pathogenic and modulating mechanisms in parasitic infections. Three major areas of research were investigated: In schistosomiasis, the role of eosinophils in the host granulomatous response and the mechanism of its production were studied. The eosinophils were found to play an important role in destruction of the schistosome eggs as in their absence infected animals sustain a more severe form of the disease and accumulate eggs in their tissues. The kinetics of granuloma formation in S. japonicum was studied as well as the chemical characteristics of its soluble egg antigens. In addition, systems of host resistance specific and non-specific were developed to evaluate the role of each component of the host immune system in killing the parasites in vitro and at the ultrastructural level. In giardiasis, methods have been developed to purfy G. muris trophozoites and to use them in an immunofluorescent system for antibody detection. The kinetics and mechanism of acquiring immunity to giardia and its loss during pregnancy and lactation has been explored. Furthermore, the infectivity of human giardia to mongrel dogs has been established. In immunology, in vitro antigen and mitogen induced 3H-thymidine incorporation was studied in peripheral blood and spleen cells of patients with schistosomiasis. The responses to nonspecific mitogens was suppressed while there was preferential preservation of the responsiveness to soluble egg antigen. The role of human peripheral blood monocytes in killing of the invading schistosomula was studied in vitro as well as the activity of murine activated macrophages. Finally, the biochemical basis for cell mediated (eosinophil and neutrophil) damage to S. mansoni schistosomula in vitro was evaluated by studying H2O2 production.